The present invention relates to an optical modulator for use in an optical communication system.
FIG. 15 is a conceptual diagram of a conventional optical modulator. As shown, the conventional optical modulator comprises an light modulating semiconductor element 1 of an electric field absorption type (hereinafter xe2x80x9can electroabsorption type light modulating semiconductor elementxe2x80x9d), a high-frequency circuit substrate 2 for feeding an RF signal to the electroabsorption type light modulating semiconductor element 1, a terminating resistor substrate 3, an input optical coupling system 7a and an output optical coupling system 7b. The terminating resistor substrate 3 includes a terminating resistor 3a for impedance matching, a through hole 3b, and a transmission line connecting them. The high-frequency circuit substrate 2, the electroabsorption type light modulating semiconductor element 1, and the terminating resistor substrate 3 are electrically connected by wires 6 or the like.
In the conventional optical modulator, the terminating resistor substrate 3 has a back side thereof serving as a grounding electrode which is electrically connected via the through hole 3b to the terminating resistor 3a. Also, the back side of the light modulating semiconductor element 1 is a grounding electrode. Hence, the light modulating semiconductor element 1 and the terminating resistor 3a are electrically connected in parallel with each other. Thus, the internal impedance of the optical modulator is a normalized impedance. The high-frequency circuit substrate 2 has a transmission line 5 for transmitting the RF signal.
The operation of the conventional modulator is now explained. For example, as the light modulating semiconductor element 1 is of an electric field absorption type, it can efficiently receive a continuously oscillated laser light from the input optical coupling system 7a. The light modulating semiconductor element 1 absorbs the laser light, and the amount of absorption is proportionate to the voltage of applied electric signal through the high-frequency circuit substrate 2. Accordingly, when high-frequency circuit substrate 2 is fed with a voltage signal for modulation, intensity of the laser light released from the emitting end of the light modulating semiconductor element 1 is modulated corresponding to the voltage of the signal. The light released from the light modulating semiconductor element 1 is efficiently provided to the output optical coupling system 7b. 
Some prior art modulators are disclosed in Japanese Unexamined Patent Publication No. Hei. 10-293278, Japanese Unexamined Patent Publication No. Hei. 10-01335 and U.S. Pat. No. 6,002,510.
However, in such an electroabsorption type light modulating semiconductor element of an electric field absorption type, its extinction characteristic, which means relationship between optical output and applied voltage, is not linear as shown in FIG. 16. More specifically, optical output of the electroabsorption type light modulating device is highly responsive to a change of the applied voltage in a lower range, while not responsive in a higher range of the applied voltage.
Accordingly, a ringing or pattern effect in the waveform of the applied voltage is emphasized and appears in the waveform of the optical output, especially at the mark side. Thus the waveform of an optical output is degraded and its eye aperture in a so-called xe2x80x9ceye patternxe2x80x9d test which evaluates the quality of waveform of optical output declines.
The present invention has been developed in view of the above drawbacks and its object is to provide an optical modulator which can produce an optical output of improved waveform while its light modulating element has a non-linear extinction characteristic.
In order to achieve the object described above, an optical modulator according to the present invention comprises an electroabsorption type light modulating semiconductor element having a non-linear extinction characteristic, a high-frequency electric circuit for supplying the light modulating semiconductor element with a high-frequency electric signal, and a terminating resistor for impedance matching, wherein the high-frequency electric circuit includes a transmission line for transmitting the high-frequency electric signal and a diode connected to the transmission line.
The diode may be connected in series to the transmission line.
The diode may also be connected in parallel with the transmission line.
A pair of diodes may be connected in parallel with the transmission line with their electrical polarities oriented opposite to each other.
Preferably, the light modulating semiconductor element and the high-frequency electric circuit may be located adjacent to each other.
The high-frequency electric circuit and the terminating resistor may be mounted integrally on a single circuit board on which the light modulating semiconductor element is also mounted in a flip-chip bonding.
A semiconductor laser emitting device may be formed and monolithically integrated with the light modulating semiconductor element.
The above and other objects, features and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate embodiments of the present invention.